1. Field of the Invention
The present invention relates to electric insulating oil compositions. More particularly, it is concerned with electric insulating oil compositions obtained by processing cracked oil containing unsaturated components with an acid catalyst.
2. Description of the Prior Art
Heretofore, purified mineral oil, synthetic hydrocarbon oil such as polybutene or alkylbenzene, chlorinated biphenyl or the like has been used as the electric insulating oil. Use of chlorinated biphenyls is now so limited due to their intoxic actions on the human body that they are no longer used as the insulating oil for the general purpose. Also, capacities of the mineral insulating oil and the synthetic hydrocarbon insulating oil are not necessarily satisfactory to meet requirements for ultrahigh-tension electric cables, high-tension and light-weight condensers and transformers. Under such circumstances, development of synthetic insulating oil superior in capacities under high-tension conditions is strongly desired.
In parallel with the higher tension for modern oil-impregnated electric cables, condensers and transformers, capacity requirements for the insulating oil are now becoming stricter, including not only a higher dielectric breakdown voltage (BDV) and a lower dielectric loss tangent (tan .delta.) but also electric characteristics in the high-voltage electric field. Insulating oil with superior characteristics in the high-voltage electric field such as stabilities toward glow discharge and corona discharge in the case of insulating oil to be impregnated in high-voltage electric instruments, for example, the electric cable, the condenser and the transformer, which possibly undergo high-tension field stress when used is now highly desirable.
Stability toward the corona discharge is determined by the corona starting voltage (CSV) and the corona ending voltage (CEV). The higher the CSV, the corona discharge is less readily discharged, and the higher the CEV, the corona discharge is more readily ended if generated. Accordingly, higher CSV and CEV are both favorable for the insulating oil.
The glow discharge or the corona discharge which occurs inside an insulator in the high-voltage electric field promotes decomposition of the insulating oil thereby causing evolution of decomposition gas which in turn not only induces dielectric breakdown but also causes destruction of the electric instrument itself fitted in the closed container such as the condenser. The gas evolved from insulating oil is mainly composed of hydrogen gas. Therefore, gaseous hydrogen absorbability of an insulating oil has a great effect upon stabilities of the insulating oil toward corona discharge and glow discharge.
On the other hand, in compliance with the abovementioned requirements for higher-voltage electric instruments, films of plastics such as polyolefins, polystyrene and polyesters, for example, polyethylene terephthalate, and particularly polypropylene films (referred to hereinbelow as PP films) have more popularly been used in place of the heretofore employed insulating paper as a part or the whole of electric instruments such as oil-impregnated electric cables and condensers.
Use of plastic films, especially those with a higher dielectric strenght, particularly PP films increases the load voltage per unit thickness of the insulator or dielectric and makes smaller or higher-voltage electric instruments available.
For the oil-impregnated electric instruments employing plastic films, an important characteristic required for the insulating oil to impregnated electric instruments is its capacility to impregnate the plastic films, in addition to BDV, tan .delta., CSV and CEV as set forth above.
In the case of insulating papers heretofore employed as the insulator or dielectric, which are aggregates of fine fibers with porous structure so that the impregnation is readily effected by capillarity, there is little difference in impregnatory capability among various kinds of insulating oils. On the contrary, impregnatory capacity of the insulating oil is critical for plastic films.
Since the electric characteristics of oil-impregnated electric instruments such as oil-impregnated electric cables and condensers are well exhibited only when the insulator or dielectric are satisfactorily impregnated with insulating oil, impregnatory capacity of the insulating oil to impregnate plastic films has a great influence upon exhibition of the capacity of an electric instrument as well as reliability of the electric instrument, for example, occurence of troubles.
Whereas heretofore used insulating oils such as mineral oil, alkylbenzene and alkylnaphthalene are practically satisfactory in BDV and tan .delta., their hydrogen-gas absorbabilities in the high-voltage electric field are not sufficiently satisfactory. Moreover, these insulating oils are unsatisfactory not only in CSV and CEV but also in capability to impregnate plastic films.
As for BDV which is a criterion for estimating the dielectric strength, it is necessary for the dielectric strength of an insulating oil impregnated in oil-impregnated electric instruments employing plastic films to be not lower than that of impregnated plastic films. If the dielectric strength of insulating oil is lower than that of plastic films, dielectric breakdown of the insulating film will occur earlier than that of the plastic films with a result that the smaller, lighter or higher-voltage electric instrument cannot be produced. In this respect, none of the heretofore employed insulating oils has a dielectric strength or BDV sufficient to meet the requirement.
As a result of extensive studies for finding a satisfactory insulating oil we have completed electric insulating oil compositions excellent not only in BDV, tan .delta. and hydrogen-gas absorbability but also in capability to impregnate plastic films.
Whereas the conventionally employed mineral oil, alkylbenzene, alkylnaphthalene and others are satisfactory in practical use for BDV and tan .delta., they are not insulating oils sufficiently absorbing hydrogen gas in high-voltage electric field.